Generally, dyeing processes for microporous polyester track etched membranes having pores with a diameter of less than 0.8 μm (so-called “small pore”) and high porosity employ disperse dyes in aqueous/alcoholic systems, followed by fixation of the dye and removal of residual dye from the membrane surfaces by reducing with sulfite-containing solutions or washing with appropriate solvents or surfactants. Due to the large number of pores with individually dyed walls, it is possible to achieve dark shades of color using this technique.
In contrast, membranes having pores wherein the largest pore size has a diameter greater to or equal to 0.8 μm (so-called “large pore”) and low porosity have relatively fewer inner pores and wall surfaces. As such, to achieve a dark shade which substantially blocks transmission at one or more wavelengths, it is essential to dye the membrane material itself much more so as compared to membranes having small pores and high porosity. Although the use of disperse dyes in solvent systems having higher boiling alcohols or other hydrocarbons may be employed to achieve this result, removal of such solvents is difficult and can negatively influence cell culture which is a common application of such membranes. Additionally, relatively high temperatures that typically exceed the glass transition temperature of the membrane material, such as polyethylene terephthalate (PET), are employed to move the dye molecules through the polymer chains to get a substantial dyeing effect. However, this approach often results in structural changes of the membrane material which can result in a change of membrane parameters such as pore diameter or surface roughness. Thus, there is a need for improved methods to achieve dark shade dyeing for microporous track etch membranes with large pores and low porosity.